If a camera is fitted with an 'auto iris lens', and the lighting is mostly from behind the subject being viewed, this can cause an over compensation resulting in a dark shadowy silhouette with a brighter area behind (also known as 'contre jour' in art and photographic circles).

If the camera is fitted with a 'Backlight Compensation' circuit, this will automatically adjust the auto iris lens setting to deliberately over expose the highlight, and therefore produce a more balanced iimage in the shadow areas. Some modern 'Digital Signal Processing' (DSP) cameras now employ very advanced multi zone sampling of the picture, to produce a more consistent and effective level of automatic override.

It's important to remember that Backlight Compensation is only possible using auto iris lenses, and cannot be enabled with manual iris optics. It perhaps goes without saying, but if you want to use high quality manual lenses, on a fixed scene with backlighting, the easiest solution is to introduce additional lighting in the foreground.It is amazing how given the amount of money spent on very high quality CCTV equipment, hardly any thought is applied to making subtle changes to room lighting, which can seriously make all the difference to the overall picture quality.

Now if you ask 100 people in the CCTV industry what benefit does increased camera sensitivity provide, 99 will probably tell you that the camera works better in the dark ....or words to that effect.

Trust Doktor Jon to have a slightly different opinion!

It is of course true to say that the more sensitive the camera, the better the quality of image reproduced under low light conditions. And yet the same principle applies in well lit environments.

Sounds strange? ... well, the simple fact is that if you want to improve the performance of a lens (lens resolution) all you need to do is close the Iris (or aperture ring), two or three 'f' stops, and the lens itself then becomes capable of resolving even more detail.

This is particulalrly useful in indoor applications, where the opportunity to use very high performance manual iris optics, on cameras fitted with an 'Electronic Iris' circuit (a "semi automatic" method of exposure control, where with a fixed iris setting on the lens, the sampling or 'shutter' speed is electronically varied [typically 1/50th > 1/100,000th of a second] to maintain the correct amount of exposure) will not only produce significantly better quality images, but also save the additional cost of using inferior performance 'auto iris' lenses.

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